UML - To be discussed since prototype is just a board game ;)

Layers

Functional Minimum:

Open World, Weapons, Player Movement, Enemies, Some type of text dialogue (Pokemon style) Fighting/survival mechanic

Low Target:

Decent story plot, survival mechanic, and end to the game, atleast 3 different levels, Shooting, Decent control scheme

Desirable Target:

Cut-Scenes, a set story-line to follow, advanced AI for enemies and NPC's

High Target:

Actual voice dialogue, Xbox capable, Even better intelligent AI, additional levels

Extras:

Player choices that affect game outcomes, DLC,  Online Multiplayer